The present invention is an improvement over a prior tool for semi-finish and finish boring piston wrist pin holes. Such prior tool comprised a "Kulite" boring bar, shrunk fit in a steel adapter, equipped with a special cartridge body having a pair of carbide inserts of square or pentagon configuration mounted for successive semi-finish and finish diameters, each with 5.degree. positive axial rake and 5.degree. clearance angle for in feed. A single central mounting cap screw retained the cartridge in a pocket milled in the bar with provision for fine adjustment of semi-finish and finish cutting diameters of the inserts through adjustment screws mounted at the ends of the cartridge.